Protective coating composition

ABSTRACT

A method is disclosed for forming a polymeric coating on a substrate surface, which method comprises the steps of activating (A) at least one monomer selected from (a) at least one polymerizable organic acid monomer comprising at least one acid group and at least one polymerizable group and (b) at least one polymerizable organic acid anhydride monomer comprising at least one acid anhydride group and at least one polymerizable group and (B) at least one polymerizable organic base monomer comprising at least one basic group and at least one polymerizable group by subjecting the monomers to a soft ionization plasma process; and depositing the activated monomers resulting from step (i) onto the substrate surface thereby forming a polymeric coating containing salts resulting from interaction between acidic and basic functional groups on side chains of the polymeric coating. Preferred polymerizable groups are alkenyl groups. Polymeric salt coatings resulting from the above method have excellent barrier properties and coatings in accordance with the present invention will enhance the hydrophilic, biocompatible, anti-fouling and controlled surface pH applications of substrates such as filtration and separations media.

CROSS-REFERENCES TO RELATED APPLICATIONS

This present application is a U.S. national stage filing under 35 USC371 and claims priority from PCT Application No. PCT/EP 03/04347entitled “PROTECTIVE COATING COMPOSITION” filed on Apr. 8, 2003,currently pending, which claims priority from Great Britain PatentApplication 0208203.0 entitled “PROTECTIVE COATING COMPOSITION” filed onApr. 10, 2002, currently pending.

FIELD OF INVENTION

The present application describes a deposition process for coatingsubstrates with a polymeric barrier coating utilizing plasma technologyand particularly relates to the deposition of barrier coatings using atleast one of polymerizable organic base monomers and polymerizableorganic acid monomers which are polymerized to form a polymeric coatingwhile maintaining their acidic or basic functionality.

BACKGROUND OF THE INVENTION

The use of polymeric salt layers as dielectric films and biodegradablecoatings have been proposed in EP 0547555 and EP 0396303 respectively.In EP 0547555 a polyimide ammonium salt reaction product of anethylenically unsaturated amine with an aromatic polyimide havingpendent carboxylic acid groups, in an organic solvent is used incombination with a cross-linker to coat substrates. In EP 0396303 amaleic acid co-polymer salt is utilized to improve biodegradability.

In EP 0376333 a process is described which utilizes plasma activatedgaseous precursors and heat to produce a polyimide thin film coating ona substrate. The polyimide forming monomers are heated to producemonomer vapors which enter a vacuum radio frequency plasma and are thenaccelerated under vacuum by an electric field to condense upon thetarget substrate. The substrate must either be heated to a temperaturein the region of about 200° C. during the coating stage or is heated toabout 200° C. once the substrate is considered to be sufficiently coatedwith ionized polyimide forming monomers, to form a polyimide thin filmon the substrate. In this cases polymerisation is affected through thereaction of acid anhydrides with diamines which results in thenon-reversible formation of imide bonds to produce polyimide structuresof the type shown below in formula (1). The free acid and free aminefunctionality of the precursors are irreversibly lost with the formationof the polyimide.

There is not the remotest suggestion in EP 0376333 that a polymer couldbe made while maintaining the acidic and basic functionalities of thepolyimide forming monomers.

It is known that gas, flavor and aroma barrier coatings can be appliedonto to substrates using acid and base precursors, as described forexample in WO 98/31719 which describes the use of a compositioncomprising ethylenically unsaturated acids such as itaconic acid and apolyamine such as polyethylenimine together with a cross-linker such asa reactive silane. The resulting composition was applied onto asubstrate in the form of a liquid coating and was then cured by means ofa free radical reaction process initiated by electron beam radiation,gamma radiation, or ultra-violet radiation.

Substrates may be coated for a variety of reasons, for example toprotect the substrate from corrosion, to provide a barrier to oxidation,to improve adhesion with other materials, to increase surface activity,and for reasons of biomedical compatibility of the substrate. A commonlyused method for modifying or coating the surface of a substrate is toplace the substrate within a reactor vessel and subject it to a plasmadischarge. Many examples of such treatment are known in the art; forexample, U.S. Pat. No. 5,876,753 discloses a process for attachingtarget materials to a solid surface which process includes affixingcarbonaceous compounds to a surface by low power variable duty cyclepulsed plasma deposition, and EP 0896035 discloses a device having asubstrate and a coating, wherein the coating is applied to the substrateby plasma polymerisation of a gas comprising at least one organiccompound or monomer. WO 01/15764 describes a multi-step method forsurface modification of a medical device involving a low temperatureplasma treatment to provide a surface of the device with a plasmadeposited layer which is then chemically treated with multifunctionallinkers which are in turn reacted with bioactive/biocompatible agents.U.S. Pat. No. 5,723,219 describes a plasma deposited film networkcomprising a plurality of radio frequency discharge plasma film layers.

WO97/38801 describes a method for the molecular tailoring of surfaceswhich involves the plasma deposition step being employed to depositcoatings with reactive functional groups, which groups substantiallyretain their chemical activity on the surface of a solid substrate,using pulsed and continuous wave plasma. Wu et al. discuss in theirrelated publication, Mat. Res. soc. Symp. Proc, vol. 544 pages 77 to 87the comparison between pulsed and continuous wave plasma for suchapplications.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention there is provided a method forforming a polymeric coating on a substrate surface, which methodcomprises the steps of

-   -   i. activating (A) a monomer selected from (a) at least one        polymerizable organic acid monomer comprising at least one acid        group and at least one polymerizable group and (b) at least one        polymerizable organic acid anhydride monomer comprising at least        one acid anhydride group and at least one polymerizable group,        and (B) at least one polymerizable organic base monomer        comprising at least one basic group and at least one        polymerizable group, by subjecting the monomers to a soft        ionization plasma process; and    -   ii. depositing the activated monomers resulting from step (i)        onto the substrate surface thereby forming a polymeric coating        containing salts resulting from interaction between acidic and        basic functional groups on side chains of the polymeric coating.

The polymerizable groups on the monomers used in the method of thepresent invention must react under soft ionization plasma conditions toform a polymer. There must be a sufficient number of groups on eachmolecule for polymerisation to occur. Hence, therefore in the case ofmonomers such as acrylic acid one vinyl group is sufficient but in somecases, at least two polymerizable groups will be required per monomerfor polymerization to occur.

Preferably, the polymerizable group of at least one of the polymerizableorganic acid and acid anhydride and the polymerizable organic base areadapted to be reactable with each other to form polymers, whilemaintaining the acidic and basic groups intact as side chains on thepolymer. The polymerizable organic acidic monomers are preferably alsoreactable with like polymerizable organic acidic monomers as well as thepolymerizable organic base monomers and similarly the polymerizableorganic base monomers are preferably also reactable with likepolymerizable organic base monomers as well as the polymerizable organicacidic monomers. Hence, preferably the polymerizable organic basemonomers and polymerizable organic acidic monomers will be randomlypolymerized together, such that polymers containing solely acidic groupsand polymers containing solely basic groups are unlikely to occur.

To obtain a coated substrate with a substantially random mix of acidicor basic group side chains, the polymerizable groups may all be the samei.e. they may all be alkenyl groups. In the case where a strictly ABABABtype polymer is required appropriate polymerizable groups may beselected such that the reactable groups on the acidic and polymerizableorganic base monomers only react by a reaction pathway. Preferably, forexample, each polymerizable groups may be an unsaturated hydrocarbongroup such as a linear or branched alkenyl group or an alkynyl group oralternatively a polymerizable group such as alkoxy group, for example,methoxy, ethoxy, propoxy, isopropoxy groups or an —OH group or the like.The polymerizable groups are preferably unsaturated hydrocarbon groupsand most preferably are alkenyl groups comprising from 2 to 10 carbonatoms such as a vinyl, propenyl, butenyl and hexenyl.

The polymerizable organic acidic monomers preferably comprise one ormore carboxylic acid groups or an acid anhydride thereof or may comprisea sulphonic or phosphonic acid group. The polymerizable organic acidicmonomers may be polybasic, or oligomers, polymers or copolymers of anunsaturated carboxylic acid or acid anhydrides. The polymerizableorganic acidic monomers may also comprise short chain co-polymers ofunsaturated carboxylic acids may be used with for example an appropriateunsaturated monomer such as ethylene, propylene, styrene, butadiene,acrylamide and acrylonitrile.

Hence, for example the polymerizable organic acidic monomers used in themethod in accordance with the present invention may be selected from oneor more of the following acrylic acid, alkylacrylic acid, fumaric,maleic, citraconic, cinnamic, itaconic acid monomethylester,vinylphosphonic acid, sorbic acid, mesaconic acid, and vinyl sulphonicacid itaconic acid, citric acid, succinic acid, ethylenediaminetetracetic acid (EDTA) and ascorbic acid.

The polymerizable organic acidic monomers may optionally contain one ormore silicon atoms therein.

The polymerizable organic base monomers may comprise any suitableorganic base having basic groups which will interact with the acidgroups referred to above to reversibly form a salt. The polymerizableunsaturated organic base may optionally contain one or more siliconatoms therein and may be polyacidic or an oligomer, polymer or copolymerof a polymerizable organic base monomers. Preferably the polymerizableorganic base monomers is a polymerizable primary or secondary amine. Thepolymerizable groups are preferably unsaturated hydrocarbon groups andmost preferably are alkenyl groups comprising from 2 to 10 carbon atomssuch as a vinyl, propenyl, butenyl and hexenyl. Most preferably thepolymerizable organic base monomer is an unsaturated primary orsecondary amine, such as for example 2-aminoethylene, 3-aminopropylene,4-aminobutylene and 5-aminopentylene.

It is to be understood that a salt resulting from the method inaccordance with the present invention is the product of the interactionbetween an acidic and a basic functional group. In the coatings producedfrom the method in accordance with the present invention, the acidic andbasic functional groups will typically exist as polymer side chains.Salt formation as described herein is the well known reversible reactionof an acid and base as shown in formula (2) below, which results in aproton exchange from the acid to the base.R—COOH+R′—NH₂

R—COO⁻+R′—NH₃ ⁺  (2)

For example therefore, an organic unsaturated acid, H2C═CRCOOH and anorganic unsaturated base, H₂C═CR′CH₂NH₂, may be reacted together underconditions of soft ionization to form a co-polymer with acidic and basicside chains of the type shown in formula (3) below. These polymers willtypically be random copolymers, although block-wise copolymers may alsobe formed.

The acidic and basic group functionality is retained subsequent topolymerization and as such the resulting co-polymer depicted in formula(3) above will typically be present in accordance with the equilibriumformula (4) below:

It will be seen from the example provided in support of the presentinvention below that, in air the coated substrate utilized had a coatingin accordance with the present invention which largely had thedisassociated structure on the right of formula (4) above and as such isdescribed therein as a polymeric ammonium carboxylate salt film.

Indeed, it should be appreciated that the equilibrium will change inaccordance with the pH environment in which the coated substrate isretained. One of the most important advantages of the present inventionis that the resulting coating may be given a predetermined acid or basicnature, in that the proportions of acid and base introduced into thelayer are such that the proportions can be determined based on therequirements for the application of interest to the user. Hence thesubstrate may be coated with any variation between a polymer resultingsolely from the polymerizable organic base monomer or a polymerresulting solely from the polymerizable organic acidic monomer asrequired or determined by the user, such that a surface of apredetermined pH may easily be applied to the substrate surface byapplying the acid and base in the required proportions which might forexample be determined through at least one simple calculation andtitration.

Optionally a further constituent may be co-reacted together with the atleast one polymerizable organic base monomer and polymerizable organicacidic monomer in the method of the present invention. This furtherconstituent is intended to function as a chain-extender or spacer(hereafter referred to as a “spacer”), and is adapted to react with thepolymerizable groups of either or both the polymerizable organic basemonomer and the polymerizable organic acid monomer so as to form part ofthe resulting polymer. The optional spacer may be any appropriatecompound providing it is able to react with the at least twopolymerizable groups of one or both of the monomers or with polymericchains formed by the monomers during the method of the presentinvention. However, when the spacer is adapted to react with either thepolymerizable group of the acid alone or the polymerizable group of thebase alone it must be reactable with a minimum of two polymerizablegroups of the polymerizable organic base monomer or a minimum of twogroups of the polymerizable organic acidic monomer respectively.

Preferably the spacer is adapted to react with the polymerizable groupsof both the polymerizable organic base monomer and the polymerizableorganic acidic monomer. Preferably the spacer is an organic compound ora reactive organosilane. Preferably, when the polymerizable groups onthe polymerizable organic basic monomers and polymerizable organicacidic monomers are unsaturated groups, the spacer comprises at one ormore alkenyl groups and therefore may comprise one or more polymerizablealkenes such as ethene, propene, butene or the like or alternatively maycomprise one or more dienes such as 1,3-butadiene, 1,4-pentadiene1,5-hexadiene, 1,6-heptadiene and 1,7-octadiene and the like.

The substrate to be coated may comprise any material, for example metal,ceramic, plastics, siloxane, woven or non-woven fibres, natural fibres,synthetic fibres cellulosic material and powder but most preferably inthe case of this invention the preferred substrate is a plasticmaterial, for example thermoplastics such as polyolefins e.g.polyethylene, and polypropylene, polycarbonates, polyurethanes,polyvinylchloride, polyesters (for example polyalkylene terephthalates,particularly polyethylene terephthalate), polymethacrylates (for examplepolymethylmethacrylate and polymers of hydroxyethylmethacrylate),polyepoxides, polysulphones, polyphenylenes, polyetherketones,polyimides, polyamides, polystyrenes, phenolic, epoxy andmelamine-formaldehyde resins, and blends and copolymers thereof.Preferred organic polymeric materials are polyolefins, in particularpolyethylene and polypropylene.

The substrate may also be of the type described in the applicant'sco-pending application WO 01/40359 wherein the substrate comprises ablend of an organic polymeric material and an organosilicon-containingadditive which is substantially non-miscible with the organic polymericmaterial. The organic polymeric material may be any of those listedabove, the organosilicon-containing additive is preferably linear orcyclic organopolysiloxanes. In the case of such substrates theorganosilicon-containing additive migrates to the surface of the mixtureand as such is available for reaction or where deemed necessary plasmaor corona treatment. It is to be understood that the term “substantiallynon-miscible” means that the organosilicon-containing additive and theorganic material have sufficiently different interaction parameters soas to be non-miscible in equilibrium conditions. This will typically,but not exclusively, be the case when the Solubility Parameters of theorganosilicon-containing additive and the organic material differ bymore than 0.5 MPa^(1/2). The present invention has particular utilityfor coating plastics and films.

The form of plasma activation utilized may be any suitable type,provided it results in a “soft” ionization plasma process. It should beunderstood that a soft ionization process is a process wherein precursormolecules are not fragmented during the ionization process and as aconsequence the resulting polymeric coating has the physical propertiesof the precursor or bulk polymer. Preferred processes are lowtemperature, cold plasmas such as low pressure pulsed plasma processingor atmospheric pressure glow discharge. Low temperature being below 200°C., and preferably below 100° C.

In the case of low pressure pulsed plasma, the acid and base arepreferably introduced into the plasma in the form of vapours andpolymerization initiated by the plasma. The low pressure pulsed plasmamay be performed with at least one of substrate heating and pulsing ofthe plasma discharge. While for the present invention heating will notgenerally be required, the substrate may be heated to a temperature upto and below its melting point. Substrate heating and plasma treatmentmay be cyclic, i.e. the substrate is plasma treated with no heating,followed by heating with no plasma treatment, etc., or may besimultaneous, i.e. substrate heating and plasma treatment occurtogether. The plasma may be generated by any suitable means such asradio frequency, microwave or direct current (DC). A radio frequencygenerated plasma of 13.56 MHz is preferred. A particularly preferredplasma treatment process involves pulsing the plasma discharge at roomtemperature or where necessary with constant heating of the substrate.The plasma discharge is pulsed to have a particular “on” time and “off”time, such that a very low average power is applied, for example of lessthan 10 W and preferably less than 1 W. The on-time is typically from 10μs to 10000 μs, preferably 10 μs to 1000 μs, and the off-time typicallyfrom 1000 μs to 10000 μs, preferably from 1000 μs to 5000 μs. Thegaseous precursors may be introduced into the vacuum with no additionalgases, however additional plasma gases such as helium or argon may alsobe utilized.

Any conventional means for generating an atmospheric pressure plasmaglow discharge may be used in the method in accordance with the presentinvention, for example atmospheric pressure plasma jet, atmosphericpressure microwave glow discharge and atmospheric pressure glowdischarge. Typically such means will employ a helium diluent and a highfrequency (e.g. >1 kHz) power supply to generate a homogeneous glowdischarge at atmospheric pressure via a Penning ionisation mechanism,(see for example, Kanazawa et al, J. Phys. D: Appl. Phys. 1988, 21, 838,Okazaki et al, Proc. Jpn. Symp. Plasma Chem. 1989, 2, 95, Kanazawa etal, Nuclear Instruments and Methods in Physical Research 1989, B37/38,842, and Yokoyama et al., J. Phys. D: Appl. Phys. 1990, 2, 374).Examples of preferred apparatus are described in the applicant'sco-pending applications WO 02/35576, which was published after thepriority date of the present application, and GB 0208261.8. The plasmais formed using pairs of electrode units. Each electrode unit containsan electrode and an adjacent dielectric plate and a cooling liquiddistribution system for directing a cooling conductive liquid onto theexterior of the electrode to cover a planar face of the electrode. Eachelectrode unit may comprise a watertight box having a side formed by adielectric plate having bonded thereto on the interior of the box theplanar electrode together with a liquid inlet and a liquid outlet. Theliquid distribution system may comprise at least one of a cooler with arecirculation pump and a sparge pipe incorporating spray nozzles. Theatmospheric pressure plasma assembly may also comprise a first andsecond pair of vertically arrayed parallel spaced-apart planarelectrodes with at least one dielectric plate between said first pair,adjacent one electrode and at least one dielectric plate between saidsecond pair adjacent one electrode, the spacing between the dielectricplate and the other dielectric plate or electrode of each of the firstand second pairs of electrodes forming a first and second plasma regionwhich assembly further comprises a means of transporting a substratesuccessively through said first and second plasma regions and is adaptedsuch that said substrate may be subjected to a different plasmatreatment in each plasma region.

It should be understood that the term vertical is intended to includesubstantially vertical and should not be restricted solely to electrodespositioned at 90 degrees to the horizontal.

For typical atmospheric pressure glow discharge plasma generatingapparatus, the plasma is generated within a gap of from 3 mm to 50 mm,for example 5 mm to 25 mm. Thus, the method in accordance with thepresent invention has particular utility for coating films, fibers andpowders when using atmospheric pressure glow discharge apparatus. Thegeneration of steady-state glow discharge plasma at atmospheric pressureis preferably obtained between adjacent electrodes which may be spacedup to 5 cm apart, dependent on the process gas used. The electrodesbeing radio frequency energized with a root mean square (rms) potentialof 1 kV to 100 kV, preferably between 4 kV and 30 kV at 1 kHz to 100kHz, preferably at 15 kHz to 40 kHz. The voltage used to form the plasmawill typically be between 2.5 kV and 30 kV, most preferably between 2.5kV and 10 kV however the actual value will depend on the chemistry andgas choice and plasma region size between the electrodes. Each electrodemay comprise any suitable geometry and construction. Metal electrodesmay be used. The metal electrodes may be in the forms of plates ormeshes bonded to the dielectric material either by adhesive or by someapplication of heat and fusion of the metal of the electrode to thedielectric material. Similarly, the electrode may be encapsulated withinthe dielectric material.

While the atmospheric pressure glow discharge assembly may operate atany suitable temperature, it preferably will operate at a temperaturebetween room temperature (20° C.) and 70° C. and is typically utilizedat a temperature in the region of 30° C. to 50° C.

When using an atmospheric pressure glow discharge system the at leastone polymerizable organic base monomer and polymerizable organic acidicmonomer may be introduced into an atmospheric pressure glow dischargeplasma as a vapor by conventional means, or as an atomized liquidaerosol. The polymeric organic acid and base materials are preferablysupplied to the relevant plasma region after having been atomised asdescribed in the applicants co-pending patent application WO 02/28548,which was published after the priority date of the present application,i.e. using any conventional means, for example an ultrasonic nozzle. Theatomizer preferably produces polymerisable monomers with drop sizes offrom 10 μm to 100 μm, more preferably from 10 μm to 50 μm. Suitableatomizers for use in the present invention are ultrasonic nozzles fromSono-Tek Corporation, Milton, N.Y., USA. The apparatus of the presentinvention may include a plurality of atomizers, which may be ofparticular utility, for example, where the apparatus is to be used toform a copolymer coating on a substrate from two differentcoating-forming materials, where the monomers are immiscible or are indifferent phases, e.g. the first is a solid and the second is gaseous orliquid.

An advantage of using an atmospheric pressure glow discharge assemblyfor the plasma treating step of the present invention as compared withthe prior art is that both liquid and solid atomized polymerizableorganic base monomers and polymerizable organic acid monomers may beused to form substrate coatings, due to the method of the presentinvention taking place under conditions of atmospheric pressure.Furthermore at least one of the polymerizable organic base monomers andpolymerizable organic acid monomers can be introduced into the plasmadischarge or resulting stream in the absence of a carrier gas, i.e. theycan be introduced directly by, for example, direct injection, whereby atleast one of the polymerizable organic base monomers and polymerizableorganic acid monomers are injected directly into the plasma.

The substrate may also be activated or pre-activated by the ionizationplasma method described above for example step (ii) occurs eithersimultaneously with or immediately after step (i) and deposition mayoccur while the substrate is in the plasma activation region.

The process gas for use in either preferred plasma treatment of themethod in accordance with the present invention may be any suitable gasbut is preferably an inert gas or inert gas based mixture such as, forexample helium, a mixture of helium and argon and an argon based mixtureadditionally containing at least one of ketones and related compounds.These process gases may be utilized alone or in combination withpotentially reactive gases such as, for example, nitrogen, ammonia, O₂,H₂O, NO₂, air or hydrogen. Most preferably, the process gas will beHelium alone or in combination with an oxidizing or reducing gas. Theselection of gas depends upon the plasma processes to be undertaken.When an oxidizing or reducing process gas is required, it willpreferably be utilized in a mixture comprising 90%-99% noble gas and 1%to 10% oxidizing or reducing gas.

The duration of the plasma treatment will depend upon the particularsubstrate and application in question.

Preferably where the method of the present invention utilizes anatmospheric plasma glow discharge plasma assembly, the means oftransporting a substrate is a reel to reel based process. Preferably insuch a case the substrate may be coated on a continuous basis by beingtransported through an atmospheric plasma glow discharge by way of areel to reel based process in which the substrate travels from a firstreel, through the first plasma region at the end of which is provided aguide means or roller or the like adapted to direct substrate which haspassed through the first plasma region into and through the secondplasma region and on to a second reel at a constant speed to ensure thatall the substrate has a predetermined residence time within therespective plasma regions. The residence time in each plasma region maybe predetermined prior to coating and rather than varying the speed ofthe substrate the length of each of plasma region may be altered suchthat the substrate may pass through both regions at the same speed butmay spend a different period of time in each due to the path length ofthe substrate through the respective plasma regions.

Optionally where required the substrate may be at least one of cleanedand activated prior to coating, using a helium or air plasma. Preferablyat least one of the cleaning and activation steps will be carried out bysubjecting the substrate to exposure to a plasma treatment.

Substrates coated by the deposition method of the present invention mayhave various utilities. In particular, it has been found that apolymeric salt coating produced in accordance with the above method hasexcellent barrier properties and coatings in accordance with the presentinvention will enhance the hydrophilic, biocompatible, anti-fouling andcontrolled surface pH applications of substrates. Controlled surface pHapplications will include filtration (both gas and liquid) andseparations media.

EXAMPLES

The invention will be more clearly understood by reference to thefollowing example with Reference to the figures in which:

FIG. 1 shows a Quantification of ammonium salt formation using N(1s) XPSanalysis

FIG. 2 shows Infrared spectra of Continuous wave and pulsed plasmadepositions a variety of compositions

EXAMPLE Polymeric Salt Coating by Low Pressure Pulsed Plasma

Acrylic acid (Aldrich, 99% purity) and allylamine (Aldrich, 99% purity)monomers were loaded into stoppered glass tubes, and further purified bymultiple freeze-pump-thaw cycles. Pulsed plasma deposition of theindividual monomers and also mixtures was carried out in a cylindricalglass reactor (418 cm³ volume) which was continuously pumped by amechanical rotary pump via a liquid nitrogen cold trap (base pressure8×10⁻³ mbar and 1.61×10⁻⁸ mol s⁻¹ leak rate). A copper coil wrappedaround the reactor was coupled to a 13.56 MHz radio frequency (RF) powersupply via an LC matching network. Prior to each experiment, the chamberwas cleaned using a 50 W air plasma at 0.3 mbar. The respective monomerfeeds were then introduced via fine control needle valves at apredetermined pressure. This was followed by ignition of the electricaldischarge and film deposition. A signal generator was used to triggerthe radio frequency (RF) supply, and the corresponding pulse waveformwas monitored with an oscilloscope. The average power <P> delivered tothe system was calculated using the following expression:<P>=P _(P) {t _(on)/(t _(on) +t _(off))}where P_(P) is the power output of the RF generator, t_(on) and t_(off)are the pulse on- and off-periods respectively, andt_(on)/(t_(on)+t_(off)) is the duty cycle (see C. R. Savage, R. BTimmons, Chem. Mater. 1991, 3, 575). Typical conditions were 10 minutesdeposition, with P_(P)=10 W, t_(on)=100 μs and t_(off)=4000 μs. Forcomparative purposes, continuous wave plasma polymer films weredeposited at 10 W. The notation used for describing plasmacopolymerization follows the sequence in which the two monomers wereintroduced into the plasma chamber and their respective pressuresettings. For example, AA_(0.2)AL_(0.1) corresponds to the introductionof 0.2 mbar acrylic acid vapour into the chamber, and then the openingup of allylamine to give a total pressure of 0.3 mbar (0.2 mbar+0.1mbar), where 1 bar is 10 ⁵Nm⁻². The polymeric films were deposited ontoglass slides (ultrasonically cleaned in a 1:1 solvent mixture ofcyclohexane/propan-2-ol) for XPS analysis, potassium bromide powder forinfrared analysis, and biaxial oriented polypropylene films (UCB) forgas permeation measurements.XPS Analysis

A Kratos ES300 electron spectrometer equipped with a Mg Kα X-ray source(1253.6 eV), and a concentric hemispherical analyser was used for XPSanalysis. Photo-emitted electrons were collected at a take-off angle of30° from the substrate normal, with electron detection in the fixedretarding ratio (FRR, 22:1) mode. XPS spectra were accumulated on aninterfaced PC computer and fitted using a Marquardt minimisationalgorithm with Gaussian peaks all having the samefull-width-at-half-maximum (FWHM). Instrument sensitivity factors usingreference chemical standards were taken as C(1s):O(1s): Si(2p):N(1s)equals 1.00:0.57:0.72:0.74.

Continuous and pulsed plasma polymerisation of the individual andmixtures of acrylic acid and allylamine monomers were compared. In thecase of salt formation, the different types of nitrogen environmentswere estimated by fitting the N(1s) XPS envelope: N—C(amine),N—C═O(amide) at 399.4-400.3 eV, and N(ammonium salt) at 401.4-401.7 eVin FIG. 1. The four plots in FIG. 1 represent the Quantification ofammonium salt formation using N(1 s) XPS analysis for the following:

-   -   (a) pulsed polyallylamine (AL_(0.3));    -   (b) pulsed plasma polymer−acrylic acid+allylamine        (AA_(0.15)AL_(0.15));    -   (c) pulsed plasma polymer−acrylic acid+allylamine        (AA_(0.2)AL_(0.1)); and    -   (d) continuous wave plasma polymer−acrylic acid+allylamine        (AA_(0.2)AL_(0.1))

The small amount of ammonium salt detected in the case of the pureallylamine pulsed plasma deposited films can be attributed topost-treatment adsorption of atmospheric CO₂. Pulsed plasmapolymerisation of AA_(0.2)AL_(0.1) monomer mixtures produced the largestamount of ammonium salt as seen in Table 1. The corresponding experimentusing continuous wave plasma conditions produced films with markedlydifferent chemical characteristics as seen in Table 1. The observedshift in N(1s) envelope towards lower XPS binding energies wasconsistent with the formation of less ammonium salt species. TABLE 1 XPSelemental composition of pulsed plasma polymer films (unless otherwisestated). % N ammo- amine/ nium % C ± % Si ± % O ± Total ± amide ± salt ±Monomer(s) 3.0 0.1 3.7 0.6 0.4 0.6 Acrylic acid 63.2 0.0 36.8 0.0 0.00.0 (AA) Allylamine 71.4 2.4 6.0 20.1 18.5 1.6 (AL) AA_(0.15)AL_(0.15)68.1 0.0 16.9 15.0 8.0 7.0 AA_(0.2)AL_(0.1) 66.9 0.0 23.3 9.8 2.5 7.3AA_(0.2)AL_(0.1) 73.2 0.0 14.8 12.0 8.7 3.3 (CW)Infra-Red Spectroscopy

Transmission infrared spectra were acquired over the 600-4000 cm⁻¹ wavenumber range at a resolution of 4 cm⁻¹ using a Mattson Polarisspectrometer. 100 scans were averaged in conjunction with backgroundsubtraction.

Infrared spectra obtained for the pulsed plasma polymer films of theindividual monomers displayed strong similarities with those reportedfor the monomers used as shown in Table 2 and FIG. 2. The infraredspectra in FIG. 2 represent the following:—

-   -   (a) acrylic acid;    -   (b) allylamine;    -   (c) acrylic acid pulse plasma polymer;    -   (d) allylamine pulsed plasma polymer;    -   (e) pulsed plasma polymer−acrylic acid+allylamine        (AA_(0.2)AL_(0.1));    -   (f) continuous wave plasma polymer−acrylic acid+allylamine        (AA_(0.2)AL_(0.1)); and    -   (g) pure acrylic acid+allylamine liquid mixture (1:1 molar        ratio).

For instance, in the case of pulsed plasma polymerised acrylic acid, thepresence of a narrow absorption band at 1720 cm⁻¹ (C═O stretch) wasindicative of high levels of carboxylic acid group retention. A broadpeak at 1638 cm⁻¹ (N—H bend) was seen for pulsed plasma depositedallylamine films. The disappearance of alkene absorption bands at1636-1642 cm⁻¹ (C═C stretch), 986-995 cm⁻¹ (trans CH=wag), and 912 cm⁻¹(CH₂=wag) correlated to the opening of the carbon-carbon double bondsduring plasma polymerisation of both monomers used.

CW and pulsed plasma deposition of AA_(0.2)AL_(0.1) mixtures gave anumber of similar infrared features, FIG. 2. The carbon-carbon doublebonds had reacted and the absorption band at 1705-1720 cm⁻¹ (C═Ostretch) characteristic of carboxylic groups (as seen for acrylic acid)was absent. Instead two new carboxylate group (salt) peaks at 1562-1576cm⁻¹ (asymmetrical CO₂) and 1391-1406 cm⁻¹ (symmetrical CO₂) wereidentified. For the pulsed plasma polymer films, these peaks were foundto be more intense relative to the methylene band at 1454-1456 cm⁻¹(thereby confirming the findings seen by XPS analysis). The infraredassignment for the carboxylate salt peak was confirmed by characterisinga 1:1 liquid mixture of acrylic acid/allylamine. TABLE 2 Assignment ofinfrared spectra. Wave number/ cm⁻¹ Assignment Symbol 1705-1720 C═Ostretching vibrations. ▪ 1599-1638 N—H bending vibrations 1636-1638Amide I band. 1636-1642 C═C stretching vibrations. ● 1638-1674 C═Nstretching vibrations. 1562-1576 Asymmetrical CO₂ ⁻ stretching ♦vibrations. 1454-1456 CH₂ bending vibrations. 1435 C—O—H bendingvibrations. 1391-1406 Symmetrical CO₂ ⁻ stretching ♦ vibrations.1244-1300 C—O stretching vibrations 986-995 Trans CH═ wagging ●  912CH₂═ wagging  831 NH₂ wagging

The polymer film growth rate was measured using a quartz crystalthickness monitor (Kronos, Inc Model QM-331) located in the centre ofthe plasma reactor.

Gas Barrier:

Gas permeation measurements were acquired using a mass spectrometryapparatus. This comprised placing a piece of coated polypropylenesubstrate between two drilled-out stainless steel flanges and a vitongasket. This assembly was attached to a UHV chamber via a gate valve(base pressure of 7×10⁻¹⁰ mbar) with the coated side of the polymer filmexposed to an oxygen (BOC, 99.998%) pressure of 1316 mbar. A UHV iongauge (Vacuum Generators, VIG 24) and a quadrupole mass spectrometer(Vacuum Generators SX200) interfaced to a PC computer were used tomonitor the permeant pressure drop across the substrate. The quadrupolemass spectrometer's response per unit pressure was independentlycalculated by introducing oxygen directly into the chamber via a leakvalve and recording the mass spectrum at a predetermined pressure of5×10⁻⁷ mbar (taking into account ion-gauge sensitivity factors). Thiswas then used to calculate mean equilibrium permeant partial pressure(MEPPP) of oxygen. Finally, the barrier improvement factor (BIF) foreach sample was determined by referencing with respect to the MEPPPvalue measured for the uncoated polypropylene film.

Oxygen gas permeation measurements showed that pulsed plasma depositionusing AA_(0.2)AL_(0.1) precursor mixtures gave rise to a ten-foldimprovement in gas barrier, Table 3. Whereas the corresponding filmprepared under continuous wave conditions produced no such improvement.TABLE 3 Oxygen permeability measurements. MEPPP Thickness DepositionRate Total Treatment Sample (10⁻⁸) BIF* (nm) (1 × 10⁻⁸ gs⁻¹) Time (min)o-PP (reference 29.1 ± 1.3 — — — — sample) pulsed deposited 18.6 ± 5.41.6 101.9 ± 2.5 0.39 133 allylamine pulsed deposited  4.3 ± 2.7 6.8253.4 ± 86.8^(†) 2.53 10 acrylic acid pulsed deposited  2.9 ± 1.8 10.0 52.1 ± 1.1 2.91 10 AA_(0.2)AL_(0.1) CW deposited 21.4 ± 3.3 1.4 102.6 ±4.0 4.34 5 AA_(0.2)AL_(0.1)*Barrier Improvement Factor^(†)Variation may be attributed to water adsorption from the laboratoryatmosphere.

Hence from the above it will be seen that the pulsed plasmaco-polymerisation of acrylic acid with allylamine leads to thedeposition of polymeric ammonium carboxylate salt films. Thesestructurally well-defined layers exhibit high resistance to gaspermeation.

1. A method for forming a polymeric coating on a substrate surface,which method comprises the steps of i. activating (A) a monomer selectedfrom (a) at least one polymerizable organic acid monomer comprising atleast one acid group and at least one polymerizable group and (b) atleast one polymerizable organic acid anhydride monomer comprising atleast one anhydride group and at least one polymerizable group, and (B)at least one polymerizable organic base monomer comprising at least onebasic group and at least one polymerizable group, by subjecting themonomers to a soft ionization plasma process; and ii. depositing theactivated monomers resulting from step (i) onto the substrate surfacethereby forming a polymeric coating containing salts resulting frominteraction between acidic and basic functional groups on side chains ofthe polymeric coating.
 2. The method in accordance with claim 1 whereinthe soft ionization plasma process is a low pressure pulsed plasma. 3.The method in accordance with claim 2 wherein the pulse on-time is from10 to 1000 μs, and pulse off-time is from 1000 to 10000 μs.
 4. Themethod in accordance with claim 1 wherein the soft ionization plasmaprocess is an atmospheric pressure glow discharge.
 5. The method inaccordance with claim 1 wherein the polymerizable organic acid monomeris a polymerizable carboxylic acid.
 6. The method in accordance withclaim 5 wherein the polymerizable carboxylic acid is selected from atleast one of acrylic acid, alkylacrylic acid, fumaric acid, maleic acid,citraconic acid, cinnamic acid, itaconic acid, sorbic acid and mesaconicacid.
 7. The method in accordance with claim 1, wherein the organic basemonomer is a polymerizable primary or secondary amine.
 8. The method inaccordance with claim 7 wherein the organic base monomer is selectedfrom at least one of 2-aminoethylene, 3-aminopropylene, 4-aminobutylene,and 5-aminopentylene.
 9. The method in accordance with claim 1, whereinthe step of activating further comprises activating a spacer molecule.10. The method in accordance with claim 9 wherein the spacer molecule isan alkene or diene.
 11. The method in accordance with claim 1, whereinthe substrate surface is activitated, or cleaned and activated using aplasma treatment before depositing the activated monomers.
 12. Themethod in accordance with claim 2 wherein at least one of thepolymerizable organic base monomer and the polymerizable organic acidmonomer is introduced into the pulsed plasma in the form of a vapor. 13.The method in accordance with claim 4 wherein at least one of thepolymerizable organic base monomer and the polymerizable organic acidmonomer is introduced into the atmospheric pressure glow discharge inthe form of an atomized liquid.
 14. The method in accordance with claim13 wherein the atomized liquid is produced using an ultrasonic nozzle.15. A substrate having a deposited coating prepared according to themethod of claim
 1. 16. (canceled)